SCHED_4BSD(9) | Kernel Developer's Manual | SCHED_4BSD(9)

# NAME

sched_4bsd — The 4.4BSD thread scheduler

# SYNOPSIS

#include <sys/sched.h>

void
resetpriority(lwp_t *l);

void
sched_tick(struct cpu_info *ci);

void
sched_schedclock(lwp_t *l);

void
sched_pstats_hook(struct proc *p, int minslp);

void
sched_setrunnable(lwp_t *l);

void
updatepri(lwp_t *l);

# DESCRIPTION

The traditional 4.4BSD scheduler employs a “multilevel feedback queues” algorithm, favouring interactive, short-running threads to CPU-bound ones.

resetpriority() recomputes the priority of a thread running in user mode. If the resulting priority is higher than that of the current thread, a reschedule is arranged.

sched_tick() gets called from hardclock(9) every 100ms to force a switch between equal priority threads.

The priority of the current thread is adjusted through sched_schedclock(). The priority of a thread gets worse as it accumulates CPU time.

sched_pstats_hook() gets called from sched_pstats() every Hz ticks in order to recompute the priorities of all threads.

sched_setrunnable() checks if an LWP has slept for more than one second. If so, its priority is updated by updatepri().

# CODE REFERENCES

The 4.4BSD scheduler subsystem is implemented within the file sys/kern/sched_4bsd.c.

# SEE ALSO

csf(9), hardclock(9), mi_switch(9), sched_m2(9), userret(9)

Marshall Kirk McKusick, Keith Bostic, Michael J. Karels, and John S. Quarterman, The Design and Implementation of the 4.4BSD Operating System, Addison Wesley, 1996.

July 14, 2007 | NetBSD 7.2